Analog capacitor in dual damascene process

ABSTRACT

A process for forming a capacitive structure that includes an upper layer having a first capacitor electrode section therein. A capacitor dielectric layer is formed adjacent the upper layer. The capacitor dielectric layer covers the first capacitor electrode section. A second capacitor electrode layer is formed adjacent the capacitor dielectric layer. The second capacitor electrode layer includes a second capacitor electrode section that at least partially covers the first capacitor electrode section, and which has an edge portion that extends beyond the underlying first capacitor electrode section. The capacitor dielectric layer being disposed between the first capacitor electrode section and the second capacitor electrode section. An upper dielectric layer is formed adjacent the second capacitor electrode section. Portions of the upper dielectric layer and the second capacitor electrode section are selectively removed to form a first via cavity that extends through the upper dielectric layer and the edge portion of the second capacitor electrode section. This exposes the edge portion of the second capacitor electrode section within the first via cavity. The first via cavity is filled with a via metal, which makes electrical connection with the edge portion of the second capacitor electrode section that is exposed within the first via cavity.

FIELD

This invention relates to the field of integrated circuit fabrication. More particularly, the invention relates to forming metal-insulator-metal capacitor structures in a dual damascene fabrication process.

BACKGROUND

Processes for forming a metal-insulator-metal capacitor in a copper dual damascene process in an integrated circuit, such as a complimentary metal oxide semiconductor structure, generally require etching a via through an intermetal dielectric layer to expose a copper top plate of the capacitor. Another via generally must be etched through the intermetal dielectric layer and through a capacitor dielectric layer to expose a copper bottom plate of the capacitor. In prior processes, the etching step to form the vias had to be carefully controlled to ensure that the etching of the via to the top plate did not break through the capacitor dielectric and into the bottom plate, thereby causing an electrical short between the top and bottom plates. To help prevent such etch-through, the top plate was made relatively thick, such as about 1000-1400 angstroms. However, thickening the capacitor top plate introduced difficulties in subsequent dual damascene process steps for forming conductor trenches.

What is needed, therefore, is a process for forming metal-insulator-metal capacitor structures wherein timing constraints on via etch steps may be relaxed, while at the same time minimizing the thickness of the top capacitor plate to simplify the dual damascene trench-formation processes.

SUMMARY

The above and other needs are met by a process for forming a capacitive structure that includes an upper layer having a first capacitor electrode section therein. A capacitor dielectric layer is formed adjacent to the upper layer. The capacitor dielectric layer covers the first capacitor electrode section. A second capacitor electrode layer is formed adjacent the capacitor dielectric layer. The second capacitor electrode layer includes a second capacitor electrode section that at least partially covers the first capacitor electrode section, and which has an edge portion that extends beyond the underlying first capacitor electrode section. The capacitor dielectric layer being disposed between the first capacitor electrode section and the second capacitor electrode section.

An upper dielectric layer is formed adjacent the second capacitor electrode section. Portions of the upper dielectric layer, and possibly the second capacitor electrode section, are selectively removed to form a first via cavity that extends through the upper dielectric layer and possibly into or through the edge portion of the second capacitor electrode section. This exposes the edge portion of the second capacitor electrode section within the first via cavity. The first via cavity is filled with a via metal, which makes electrical connection with the edge portion of the second capacitor electrode section that is exposed within the first via cavity.

Because the edge portion of the top electrode layer does not overlie the bottom electrode layer, it is not necessary to stop the via etch exactly at the top surface of the top electrode layer, as is typically required. Thus, the typical constraints in regard to etch selectively when drilling the vias through the dielectric layer overlying the top electrode are no longer applicable. This allows for a more robust integration of capacitor processing with dual damascene copper processing. Further, because the top electrode layer does not need to function as an etch stop for the via drilling process, the top electrode layer can be of any material desired that is compatible with the other processing constraints and desired functions. In addition, the top electrode layer may be formed at a thickness that is much thinner than prior art top electrode layers that needed to function as an etch stop. Further, additional layers adjacent or near the top electrode layer, which function only or primarily as etch stops, are also no longer required.

In another aspect, the invention provides an integrated circuit having a capacitor formed according to the process described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein:

FIGS. 1A-1J depict a metal-insulator-metal capacitor structure formed using a dual damascene process according to a preferred embodiment of the invention,

FIGS. 2A-2B depict a dual damascene process flow according to a preferred embodiment of the invention,

FIG. 3 depicts a metal-insulator-metal capacitor structure formed using a dual damascene process according to an alternative embodiment of the invention, and

FIGS. 4A-4B depict a metal-insulator-metal capacitor structure formed using a dual damascene process according to another alternative embodiment of the invention.

DETAILED DESCRIPTION

Referring now to FIGS. 1A-1J and FIGS. 2A and 2B, the steps of an integrated circuit fabrication process according to a preferred embodiment of the invention are generally depicted. Described below are the major steps in the process according to the invention. Other steps not described in detail herein may also be required to complete the processing, such as photoresist removal and rinsing steps.

The steps of the process described below are preferably performed on a structure such as depicted FIG. 1A. This starting structure preferably includes layers L1, L2, and L3, as described below. Layer L1 preferably includes metal sections M1 ₁ and M1 ₂ separated by a dielectric region ILD1. Layer L3 preferably includes metal sections M2 ₁ and M2 ₂ separated by a dielectric region ILD2. The metal sections M1 ₁, M1 ₂, M2 ₁, and M2 ₂ may be formed from various metals, such as copper or aluminum, but are most preferably formed from copper. The dielectric regions ILDI and ILD2 are preferably formed from silicon dioxide or a low dielectric constant (low k) insulating layer. Between layers L1 and L3 is a dielectric layer L2, such as silicon nitride or silicon carbide. As depicted in FIG. 1A, the metal sections M1 ₁ and M1 ₂ may be connected to the metal sections M2 ₁ and M2 ₂ respectively by metal vias V1 ₁ and V1 ₂ respectively. The metal section M2 ₂, which serves as a capacitor plate of the metal-insulator-metal capacitor formed according to the invention, is also referred to herein as the first capacitor electrode section or the capacitor bottom plate.

It is appreciated that the structure shown in FIG. 1A is merely an example of a structure on which the steps according to the invention may be performed. Thus, the scope of the invention is not limited by the structure on which the steps of the process are performed.

As shown in FIG. 1B, a dielectric layer L4, also referred to herein as a capacitor dielectric layer, is formed adjacent the layer L3 (step 100 of FIG. 2A). The layer L4, which is most preferably silicon nitride (Si₃N₄), may be formed by various processes, such as sputtering or chemical vapor deposition. As discussed in more detail hereinafter, the layer L4 serves as a dielectric separator between two conductive plates in a metal-insulator-metal capacitor formed according to the invention. Thus, the layer L4 preferably has certain dielectric properties, such as breakdown voltage and dielectric constant, that are compatible with the desired characteristics of the capacitor to be formed. Although silicon nitride provides these desired properties, other similar materials having these preferred properties may also be used. Thus, the present invention is not limited to any particular material for the capacitor dielectric layer L4.

Although the invention is not limited to any particular thickness, the preferred thickness of the layer L4 is most preferably about four hundred angstroms. The thickness of the dielectric layer L4, which tends to have an affect on the electrical characteristics of the metal-insulator-metal capacitor, is preferably controlled by the process of depositing the layer L4 and by processes to which the layer L4 is exposed during subsequent processing steps, as discussed hereinafter.

With continued reference to FIG. 1B, a conductive layer L5, also referred to herein as a second capacitor electrode layer, is formed adjacent the layer L4 (step 102). The layer L5, which is most preferably titanium nitride (TiN), may be formed by various processes, such as sputtering or chemical vapor deposition. The preferred thickness of the layer L5 is about 250 angstroms. In alternate embodiments, the layer L5 may be formed from titanium, tantalum, or tantalum nitride, or other materials having properties consistent with the functions of the layer L5 as described herein.

The structure as shown in FIG. 1B is patterned, such as by covered with photoresist, patterned, and etched to remove portions of the layer L5, thereby forming the structure shown in FIG. 1C (step 104). The remaining section of the layer L5 is referred to herein as the second capacitor electrode or top plate TP. As shown in FIG. 1C, the top plate TP includes one or more portions EP which extend beyond the edge of the underlying metal section M2 ₂. Note that the capacitor dielectric layer L4 can remain intact after the etching of the layer L5.

As depicted in FIG. 1D, an inter-metal dielectric layer L6 is formed over the capacitor dielectric layer L4 and the capacitor top plate TP (step 106). The layer L6, which is most preferably silicon dioxide (SiO₂) or a low k dielectric layer, may be formed by various processes, such as spin-on coating or chemical vapor deposition.

As shown in FIG. 1E, photoresist is applied and patterned (step 108) to form a first mask layer ML1, also referred to herein as a via mask. Preferably, the via mask ML1 is patterned according to standard photolithography processing to leave photoresist over portions of the layer L6 which are to remain after completion of the etching step described below.

The structure as shown in FIG. 1E is exposed to an etchant to remove portions of the inter-metal dielectric layer L6 and portions of the top plate TP, thereby forming the structure shown in FIG. 1F (step 110). As shown in FIG. 1F, removal of portions of the layer L6 and the top plate TP leaves via cavities VC₁, VC₂, and VC₃ through which the underlying capacitor dielectric layer L4 may be exposed. This etch step may also remove portions of the capacitor dielectric layer L4. However, as discussed in more detail below, removal of some or all of the thickness of the capacitor top plate layer L5 or the capacitor dielectric layer L4 within the via cavities VC₁, VC₂, and VC₃ is acceptable. Thus, the timing of the duration of the via etch is not as critical as would otherwise be necessary if it were desired to stop the etch on the layer L4.

In the preferred embodiment, the via mask layer ML1 is removed (step 112), and a second application of photoresist is made. Preferably, the photoresist is patterned according to standard photolithography processing to form a trench mask layer ML2 (step 114), as shown in FIG. 1G. The trench mask layer ML2 is preferably patterned to leave photoresist over portions of the intermetallic dielectric layer L6 which are to remain after a subsequent etching step. Other portions of the layer L6 are left exposed, as shown in FIG. 1G. In the preferred embodiment, the trench mask ML2 also leaves the dielectric layer L4 exposed at the bottom of the via cavities VC₁, VC₂, and VC₃.

The structure as shown in FIG. 1G is exposed to an etchant to remove portions of the inter-metal dielectric layer L6 and to remove remaining portions of the capacitor dielectric layer L4 (step 116) at the bottom of the via cavities VC₁, VC₂, and VC₃. After removal of the trench mask layer ML2 (step 118), the structure appears as shown in FIG. 1H. The etching may be done with a single mechanical etch, such as a sputter etch, or with an etchant that does both etching steps but preferably does not etch copper. As shown in FIG. 1H, removal of the portions of the layer L6 leaves trenches T₁, T₂, and T₃ adjacent the via cavities VC₁, VC₂, and VC₃, respectively.

Removal of the portion of the capacitor dielectric layer L4 at the bottom of the via cavity VC₁ extends the cavity VC₁ down to the underlying metal section M2 ₁ of the layer L3. As shown in FIG. 1H, the etching of the portion of the capacitor dielectric layer L4 at the bottom of the via cavities VC₂ and VC₃ may completely punch through the capacitor dielectric layer L4. However, this is an acceptable situation, as described in more detail below.

As depicted in FIG. 1I, metal, such as copper, aluminum, or another compatible electrically conductive material is applied to the structure to form a third metal layer L7 (step 120). The metal layer L7, which is preferably copper, may be formed by electroplating, chemical vapor deposition, sputtering, or another deposition process consistent with the materials, processes, and structures described herein. Thus, it is appreciated that the scope of the invention is not limited by the type of metal used in the layer L7, or its method of metal application.

In the preferred embodiment of the invention, the structure is planarized, such as by chemical-mechanical polishing (step 122), to provide the structure as depicted in FIG. 1J. Chemical-mechanical polishing is the preferred method of planarization. The portions of the metal layer L7 disposed in the trenches T₁, T₂, and T₃ after the planarization step are referred to herein as electrodes M3 ₁, M3 ₂, and M3 ₃, respectively. The planarization step preferably removes any metal that has bridged the layer L6 between the electrodes M3 ₁, M3 ₂, and M3 ₃, thereby electrically isolating the electrodes M3 ₁, M3 ₂, and M3 ₃. The portions of the metal layer L7 disposed in the via cavities VC₁, VC₂, and VC₃ are referred to herein as vias V2 ₁, V2 ₂, and V2 ₃, respectively.

As depicted in FIG. 1J, even though the metal in the vias V2 ₂ and V2 ₃ extends below the capacitor dielectric layer L4, there is no shorting of the vias V2 ₂ and V2 ₃ to the capacitor bottom plate M2 ₂ because the regions EP of the top plate TP through which the vias V2 ₂ and V2 ₃ extend are beyond the edges of the bottom plate M2 ₂. Electrical continuity between the vias V2 ₂ and V2 ₃ and the top plate TP is established around the periphery of the vias V2 ₂ and V2 ₃.

Since the current carried by the vias V2 ₂ and V2 ₃ is preferably alternating current (AC) having no direct current (DC) component, most of the current is carried at the outer surfaces of the vias V2 ₂ and V2 ₃. Thus, the fact that the vias V2 ₂ and V2 ₃ do not stop at the upper surface of the top plate TP, such that central portions of the vias V2 ₂ and V2 ₃ do not contact the top plate TP, does not significantly affect the electrical resistance of the connection between the top plate TP and the vias V2 ₂ and V2 ₃. Further, if lower connection resistance between the top plate TP and the electrodes M3 ₂ and M3 ₃ is desired, more vias between the electrodes M3 ₂ and M3 ₃ and the top plate TP may be incorporated. For example, there may be one via per each ten picofarads of capacitance of the resultant metal-insulator-metal capacitor formed by the process.

Although FIG. 1J depicts vias V2 ₂ and V2 ₃ making contact at each end of the top plate TP, it should be appreciated that electrical contact need not be made at both ends, but may be made at only one end or the other. Thus, embodiments of the structure may include via V2 ₂ but not via V2 ₃, or via V2 ₃ but not via V2 ₂.

According to the previously described embodiment as shown in FIG. 1J, electrical contact to the capacitor bottom plate M2 ₂ is made at the bottom of the structure through via V1 ₂ and metal section M1 ₂. In an alternative embodiment of the invention as depicted in FIG. 3, electrical contact to the capacitor bottom plate M2 ₂ is made at the top of the structure through via V2 ₃ and metal section M3 ₃. Note that in the embodiment of FIG. 3, the bottom plate M2 ₂ extends beyond the edge of the top plate TP, and that the via V2 ₃ contacts the underlying bottom plate M2 ₂ but not the top plate TP. Except for the differences in the positioning of the top plate TP with respect to the bottom plate M2 ₂, the process steps as described above are also preferably applied in fabricating the structure shown in FIG. 3.

An alternative embodiment of the process is performed on a structure such as depicted FIG. 4A. This starting structure is generally the same as that depicted in FIG. 1A, except layer L3 includes metal sections M2 ₃ and M2 ₄ on either side of the metal section M2 ₂. The metal sections M2 ₃ and M2 ₄, which are electrically separated from the section M2 ₂ by the dielectric region ILD2, are preferably positioned directly under the location where the vias V2 ₂ and V2 ₃ will be formed. Thus, during the etching of the vias V2 ₂ and V2 ₃ (step 110), if the etching operation punches through the capacitor dielectric layer L4, the bottoms of the vias V2 ₂ and V2 ₃ contact the metal sections M2 ₃ and M2 ₄, respectively, instead of the dielectric material ILD2. Otherwise, the process steps as described above are also preferably applied in fabricating the structure shown in FIG. 4B.

This embodiment addresses situations in which the etchant for the via cavities VC2 ₂ and VC2 ₃ provides poor selectivity between the dielectric material ILD2, such as low-k silicon dioxide, and the materials of L4, L5, and L6. In such a situation the dielectric material ILD2 would tend to pit, making deposition of copper within the via cavities VC2 ₂ and VC2 ₃ relatively difficult. In such a situation the bottoms of the via cavities VC2 ₂ and VC2 ₃ can be filled with a barrier and copper seed. Alternately, the structures M2 ₃ and M2 ₄ of FIG. 4B overcome this issue.

The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled. 

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 11. An integrated circuit including a capacitive structure that includes an upper layer having a first capacitor electrode section therein, formed according to the process of: (a) forming a capacitor dielectric layer adjacent the upper layer, the capacitor dielectric layer covering the first capacitor electrode section, (b) forming a second capacitor electrode layer including a second capacitor electrode section adjacent the capacitor dielectric layer, the second capacitor electrode section at least partially covering the first capacitor electrode section and having an edge portion extending beyond the underlying first capacitor electrode section, the capacitor dielectric layer disposed between the first capacitor electrode section and the second capacitor electrode section, (c) forming an upper dielectric layer adjacent the second capacitor electrode section, (d) selectively removing portions of the upper dielectric layer to form a first via cavity extending through the upper dielectric layer and at least to the edge portion of the second capacitor electrode section, thereby exposing the edge portion of the second capacitor electrode section within the first via cavity, and (e) filling the first via cavity with a via metal, the via metal making electrical connection with the edge portion of the second capacitor electrode section exposed within the first via cavity.
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 20. An integrated circuit, the improvement comprising a metal insulator metal capacitor having a top plate, a bottom plate, and a dielectric capacitance layer disposed between the top plate and the bottom plate, the top plate offset horizontally from the bottom plate such that portions of the top plate do not overlie portions of the bottom plate.
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 22. The integrated circuit of claim 20, further comprising copper vias formed with a dual damascene process, the copper vias for making electrical connections to the top plate, where the copper vias extend through the portions of the top plate that do not overlie portions of the bottom plate.
 23. The integrated circuit of claim 20 wherein the top plate further comprises a layer of titanium nitride with a thickness of about 250 angstroms. 